An optical modulator is used in many optical information communication systems. Thus, it is useful to understand characteristics of an optical modulator. Parameters specifying performance of an optical modulator include the insertion loss, modulation index, half-wavelength voltage (Vπ), optical band, ON/OFF extinction ratio, polarization extinction ratio, and chirp parameter. The method for evaluating characteristics of an optical modulator has been investigated and some evaluation methods have been reported.
Japanese Patent No. 3538619 discloses an invention of determining the modulation index by using a measured power spectrum after measuring the power spectrum of an MZ-type optical modulator having one Mach-Zehnder interferometer (MZI).
Japanese Patent No. 3866082 discloses an invention of determining the half-wavelength voltage and chirp parameter of an optical modulator from a spectral distribution of an MZ-type optical modulator having one MZI.
On the other hand, optical modulators containing a plurality of MZIs have recently been developed. It is desirable to evaluate characteristics of individual MZIs in such an optical modulator. However, individual MZIs in such an optical modulator cannot be evaluated appropriately according to a conventional method for evaluating characteristics of an optical modulator having one MZI.    Patent Literature 1: Japanese Patent No. 3538619    Patent Literature 2: Japanese Patent No. 3866082